1. Field of the Invention
The present invention relates to a polishing plate for polishing an optical fiber connector ferrule end face mounted to a joining part in an optical fiber connector for mechanically connecting optical fibers, and to a constant range control polishing apparatus and a constant pressure control polishing apparatus for the optical fiber connector ferrule end face.
2. Description of the Prior Art
In contrast to fusion splicing for permanently connecting optical fibers, an optical connector is a joining device capable of detachably connecting optical fibers with one another, and there have heretofore been used various types of optical connectors which can easily connect and disconnect optical fibers.
For example, there has been known a single-core optical connector which is intended to achieve connection of optical fibers with reduced eccentricity or angular deviation (e.g. an eccentricity of not more than 1 mm in the case of a ferrule for a single mode optical fiber having a core diameter of 10 mm), and which includes ceramic-made ferrules (reinforcing cylindrical rods for joining), optical fibers whose end positions are inserted and fixed in the ferrules, respectively, and a split sleeve having a precise inner diameter and a slit along the length thereof. The optical connector ferrules are inserted from both ends thereof in the slit sleeve and abut each other, the split sleeve being tightened together with the two ends of the optical connector ferrule.
The optical connector, which has good connection characteristics, is widely used in large amounts, especially in the optical communication field.
The above optical connector is of a type in which the optical connector ferrules are directly abut each other. To improve the optical connection characteristics in these optical connectors, use is made of optical connector ferrules each end of which has a convex-curved surface. The convex-curved shape is formed by polishing. Therefore, the polishing is required to be done with a high precision, and improved workability and operability are needed in the polishing work.
To polish the end face of an optical connector ferrule to form a convex-curved surface, a three-step polishing process as shown in FIG. 1A to FIG. 1D is required. Specifically, initially before polishing, as shown in FIG. 1A, an end face 211 of the ferrule 210 is a flat surface, and a fiber 220 and an adhesive 230 are protruded from the end face 211. End face 211 is polished so as to yield a convex-curved surface by a three-step polishing process shown in FIG. 1B to FIG. 1D. In a first step shown in FIG. 1B, the protruded parts 220 and 230 are removed (hereinafter referred to as adhesive removal) to yield a flat end face. In a second step shown in FIG. 1C, the end face 211 of the flat surface ferrule 210 is polished to yield a convex-curved surface (hereinafter this step being referred to as rough polishing). In a third step shown in FIG. 1D, surface irregularities or a processing strain layer 221 is removed by polishing (hereinafter this step being referred to as final polishing).
Since such a convex-curved shape is achieved by polishing in the current optical connector production, a high-precision polishing machine with improved workability and operability is required.
For ease of understanding, some examples of conventional polishing machines will be explained with reference to FIGS. 2, 3A, 3B, and 4.
Prior Art Example 1
FIG. 2 is a schematic cross sectional view showing a polishing machine according to a first prior art example. In FIG. 2, a polishing machine 1001 is provided for polishing the end face 211 of the optical connector ferrule 210 (hereinafter simply referred to as ferrule) to have a convex-curved shape. The polishing machine 1001 comprises a polishing plate 310 having a concave-curved polishing surface 311, and a ferrule holder 320 mounted thereon. The ferrule holder 320 holds at least three ferrules 210 (only two being shown) arranged perpendicularly with respect to the concave-curved surface 311 of the polishing plate 310. In polishing, the ferrule holder 320 undergoes precession movement utilizing the weight of the ferrule holder 320 as a polishing pressure while supplying the polishing surface 211 of the rotating polishing plate 310 with an abrasive liquid 330, thereby polishing the ferrule end face 211.
Prior Art Example 2
FIG. 3A shows a schematic cross sectional view of a polishing machine according to a second prior art example, and FIG. 3B a schematic cross sectional view showing the ferrule being attached to the polishing machine shown in FIG. 3A. As shown in FIG. 3A, a polishing machine 1002 has a rotary polishing surface 340, which is provided with a resin film 350 under a constant tension. The polishing machine 1002 polishes a ferrule 210 from which adhesive 230 is previously removed. Further, before polishing, as shown in FIG. 3B, it is necessary that the end face 211 of the ferrule 210 be pressed against a block 370 previously placed at a different position while the ferrule 210 is being held by a chuck 360 so that a concave set of the end face 211 of the ferrule 210 into the film 350 at the polishing position is a constant value L. In polishing, the ferrule 210 is rotated in the forward and reverse sense about its axis while supplying the abrasive liquid 330 on top of the rotating constant-tension film 350, and is further given a precession movement to move the contact position with the film 350.
Prior Art Example 3
FIG. 4 is a schematic cross sectional view showing a polishing machine according to a third prior art example. In FIG. 4, a polishing machine 1003 has a rotary polishing plate 380, which is provided with a polishing sheet 392 or the like on the top surface of an elastic member 391 such as rubber. The ferrule 210, mounted on a ferrule mounting piece 400 is pressed by a spring 410 against the polishing sheet 392. In polishing, the end of the ferrule 210 is pressed against the polishing plate 380 while supplying the abrasive liquid 330 on the top surface of the rotating and swinging polishing plate 380, thereby polishing the end face 211 of the ferrule 210.
The above described polishing machines 1001, 1002, and 1003 of the first to third prior art examples present problems as will be explained below.
When polishing is performed using the first prior art example polishing machine 1001, there must be used a plurality of polishing machines 1001 with different polishing surfaces 311 for the individual steps. Therefore, each step requires mounting and removing the ferrule 210 with respect to the holder 320, thus requiring troublesome work. Further, use of a plurality of polishing machines is disadvantageous in terms of cost.
In polishing with the second prior art example polishing machine 1002, since the adhesive must be removed manually using an adhesive removing tool or the like, the machine is poor in workability. Further, in the rough polishing steps, and final polishing using abrasive liquids 330 of different particle sizes, separate polishing machines must be used in order to prevent the abrasive liquids 330 from mixing with one another. Therefore, this machine requires a troublesome amount of work associated with mounting and removing the ferrule 210 with respect to the polishing machines. Further, use of two polishing machines results in an increased costs for the polishing process.
In polishing by the third prior art example polishing machine 1003, a plurality of polishing plates 380 differing in particle size must be used for the individual steps. Further, the polishing plate 380 must be exchanged every time the process proceeds to a different step. Normally, in this type of polishing machine, in order to reduce the time for rough polishing, the rough polishing step is further divided into two to three substeps in which a set of polishing sheets 392 with different particle sizes are used to polish the ferrule 210. Therefore, it is necessary to use the polishing plates 380 with different particle sizes for the individual substeps, thus requiring a troublesome amount of work.
Moreover, in view of polishing accuracy, the prior art polishing machines have involved problems as will further be explained below.
When polishing with the first prior art example polishing machine 1001, it is necessary to set constant relative distances between the respective end faces 211 of the ferrules 210 and the polishing surface 311 of the polishing plate 310 to prepare precise convex-curved surfaces uniformly; variations in length of the ferrules 210 mounted on the ferrule holder 320 largely affect the relative distances and cause polishing errors and adversely effect the polishing precision. More specifically, the length of each ferrule 210 inherently has a production error of several tens of .mu.m in the longitudinal direction even in the initial condition, and a once-polished ferrule has a maximum polishing error of 0.1 to 0.2 mm which causes problems in repolishing. Further, it is necessary to mount manually the ferrule 210 to the ferrule holder 320 using a special tool in order to flush the end positions of three or more ferrules 210, requiring a troublesome amount of work during polishing.
When polishing with the second prior art example polishing machine 1002, since normally a curvature radius of the end face 211 of the ferrule 210 of approximately 20 mm is appropriate in view of optical connection characteristics of the optical connector, the polishing machine 1002 is designed such that the curvature radius of the end face 211 of the ferrule 210 can be controlled by specifying the concave set of the end of the ferrule 210 in the film 350. FIG. 5 is a graph illustrating the relationship between the concave set in the polishing machine and the curvature radius. From FIG. 5, it can be seen that if the concave set of the curvature ferrule 210 is varied by only 50 .mu.m, the radius of the end face 211 is varied by 5,000 .mu.m. Further, the concave set of the ferrule 210 has a very small dimension, on the order of only several hundred .mu.m.
Therefore, even with an end position error of the ferrule 210 of several tens of .mu.m, a concave pressure onto the film 350 is varied, which directly results in a change in the curvature radius of the end face 211. Therefore, to obtain a radius of the end face 211 for achieving a good optical connection condition, it is necessary to control a very small concave set of about 0.3 mm with a precision of several tens of .mu.m. Therefore, where manual work must be done for holding the ferrule 210 on a chuck 360 on the block 370, extreme care must be used to tighten the chuck 360 so that the ferrule 210 is not dislocated, which requires a troublesome amount of work in polishing under conditions where the polishing precision is deteriorated.
Also the third prior art example polishing machine 1003 is designed so that the radius of the end face 211 of the ferrule 210 can be controlled by specifying the concave set of the end face 211 of the ferrule 210 into the elastic member 391. Therefore, as in the polishing machine 1002, even an end position error of several tens of .mu.m of the ferrule 210 varies the concave pressure into the elastic member 391, leading directly to a change in the radius of the end face 210. Therefore, to obtain a radius of the end face 211 for achieving a good optical connection condition, it is necessary to control a very small concave set of about 0.3 mm with a precision of several tens of .mu.m. However, since the polishing machine 1003 generates the concave set of the ferrule 210 by a deflection force of the spring 410, the deflection force tends to be varied when the end position of the ferrule 210 has an error as described above, and, hence, the variation in the deflection force results in variation in the curvature radius.